The present invention relates to a deflection optical element, an optical recording head and an optical recording apparatus.
In recent years, it has been increasingly demanded that an information recording medium should have a high density recording capability, and according to the abovementioned trend, various kinds of recording methods for coping with such the demand have been proposed. The heat assistance magnetic recording method can be cited one of such the recording methods as abovementioned. In order to achieve the high density recording capability, it is necessary to make a dimension of an individual magnetic domain as smaller as possible, while a material having a strong magnetic coercive force is employed in order to store data in a stable manner. With respect to such the recording medium as abovementioned, it is necessary to generate a strong magnetic field at the time when the data is written into the recording medium. However, the strength of the magnetic field to be generated by a small-sized head, which is fabricated so as to correspond to the minimized magnetic domain, has been limited.
To overcome the abovementioned problem, there has been proposed such a method for recording a bit onto a recording spot in such a state that the magnetic coercive force is weakened by regionally heating the recording spot at the time of recording so as to generate a magnetic softening phenomenon at that spot, and then, stopping the heating operation to naturally cool the recording spot, in order to guarantee the stability of the magnetized bit recorded on the recording spot concerned. The abovementioned method is called the heat assistance magnetic recording method.
In the heat assistance magnetic recording method, it is preferable that the operation for heating the recording spot on the recording medium is completed instantaneously. Further, it is not allowed for the heating mechanism and the recording medium to physically contact each other. Owing to this restriction, the heating operation is generally achieved by using the light absorbing action, and the method in which the light is employed for the heating operation is called the light assistance method. In order to conduct the high density recording operation by employing the light assistance method, it is necessary to yield a microscopic light spot whose diameter is equal to or smaller than the wavelength of the light concerned.
To achieve the abovementioned goal, there has been employed such a optical head that utilizes the near field light (also referred to as the near visual field light), which is to be emitted from an optical aperture whose diameter is equal to or smaller than the wavelength of the incident light. Further, there has been proposed such the optical recording head, the light condensing efficiency of which is high, while having a capability of yielding a microscopic light spot as abovementioned. With respect to such the optical recording head as abovementioned, the U.S. Pat. No. 6,944,112 sets forth the following configuration.
The optical recording head is provided with a writing magnetic pole and a planar waveguide having a core layer and cladding layer located near the writing magnetic pole. The core layer is provided with at least an edge that is shaped in a parabolic line to reflect a magnetic wave within the core layer concerned, so as to guide the magnetic wave onto the focal point. Further, a leading edge portion, at which the focal point of the parabolic line is located, is formed in a flat surface shape as if the leading edge portion of the parabolic line were cut off. This leading edge portion is disposed near the air bearing surface (also referred to as an ABS: an Air Bearing Surface) at which the recording head and the recording medium oppose to each other.
Further, the core layer is provided with a diffraction grating to introduce light into the core layer concerned. For instance, when a collimated laser beam is irradiated onto the diffraction grating at a predetermined incident angle, the collimated laser beam is efficiently focused onto the core layer so as to converge onto the focal point located near the leading edge portion. The light emitted from the abovementioned leading edge portion is irradiated onto the recording medium so as to heat the recording medium.
On the other hand, with respect to the MO (Magnet-Optic) recording head, the principle of which is similar to that of the heat assistance magnetic recording head, in order to make the MO recording head formed in a thin shape, a certain type of MO recording head employs a reflection type diffraction grating that is provided with a converting function between convergence light or divergence light and parallel light, a function for bending the optical axis and a function for correcting an anisotropy of the light intensity distribution emitted by the laser diode, as an element that deflects the laser beam emitted by the laser diode. The reflection type diffraction grating abovementioned converts the diverging light, shaped in an ellipse, to the parallel light shaped in a circle, as set forth in Tokkaihei 5-325244 (Japanese Patent Application Laid-Open Publication).
With respect to the planar waveguide set forth in U.S. Pat. No. 6,944,112, when the parallel light, having a circler light-bundle cross sectional shape, being orthogonal to the optical axis thereof, enters into the diffraction grating that converges the parallel light emitted from the light source at the predetermined incident angle, the cross sectional shape of the incident light at the incident surface of the diffraction grating is the ellipse whose long axis is extended in a direction indicating the incident angle.
For instance, when the light bundle having the ellipse shape enters into the diffraction grating provided in the planar waveguide, as abovementioned, every kind of optical aberrations would be getting large. Specifically, since the spherical aberration and the field curvature become large in the deflecting direction in principal, the efficiency degradation becomes considerable level. Accordingly, owing to the influences of the abovementioned aberrations, when the light, focused onto the core layer by the diffraction grating, propagates within the core layer concerned so as to converge the light onto the focal point located near the leading edge portion, it becomes difficult to form such a light spot that is sufficiently small.
Further, according to the reflection type diffraction grating set forth in Tokkaihei 5-325244, by making the parallel light, shaped in a circular spot, enter onto the reflection type diffraction grating concerned, it becomes possible to make the reflected light to be formed in an ellipse shape having a short axis extended in a deflecting direction, due to the regression characteristic of the light. However, the deflecting direction is substantially the right angle, and the reflected light is the converged light. Therefore, even if the reflection type diffraction grating abovementioned is employed for the optical recording head set forth in U.S. Pat. No. 6,944,112, it is impossible to make the aberrations of the light, which is to be entered into the diffraction grating, small, so as to form a sufficiently small light spot by using the planar waveguide. Still further, although Patent Document 1 sets forth such a possibility that the thickness of the MO recording head is made to be thin by setting the inclination of the reflection type diffraction grating at an angle of 45° or smaller, since the deflection angle is substantially the right angle, optical elements, which are to be disposed on the optical path of the later stage after deflected in the optical system, would be factors for increasing the thickness of the MO recording head. As a result, one can hardly ask for making the thickness of the MO recording head thinner than ever.